The invention relates to an apparatus for separation of a liquid flow flowing through a pipeline into a light fraction and a heavier fraction, in which the fluid flow is set into rotation so that it is separated into a central zone essentially containing the light fraction, and an outer annular zone essentially containing the heavier fraction, and from which the fluid in the central zone and the fluid in the outer zone are discharged via respective outlet means.
In offshore extraction of hydrocarbons (oil and gas) the produced fluids are often transported in relatively long pipelines and risers up from the sea bed to the relevant production platform. The produced fluid usually consists of liquid (oil and water) in which gas is dissolved Since the pressure in the underground source is higher than in the pipeline, and in addition decreases upwards in the pipeline, an increasing gas bubble formation and thereby two-phase flow arises in the pipeline. This implies a pulsatory flow (so-called slugging) which in many cases causes serious vibrations in the pipeline.
It is known to separate the fluid flow into liquid and gas by utilising cyclone techniques as stated in the introduction, to reduce the vibrations resulting from gas bubbles. However, the known cyclone techniques imply turbulent pressure variations in the fluid, something which causes vibrations in the outlet pipe for the gas. These vibrations may result in considerable damage, and the separation degree may also be influenced in an unfavourable manner because of the pressure vibrations.
A general object of the invention is to provide an apparatus which is able to separate a fluid flow in a light fraction and a heavier fraction by utilising xe2x80x9cin linexe2x80x9d technology in a pipeline to carry out the relevant separation work.
A more particular object of the invention is to provide such an apparatus which is suitable for efficient in-line separation of a fluid flow into a gas phase and a liquid phase, so that the flow regime in the pipeline changes from a serious pulsatory flow (slugging) to an easily manageable bubble flow.
An additional object of the invention is to provide such an apparatus which is provided with a control system securing an optimum separation operation of the apparatus.
For achieving the above-mentioned objects there is provided an apparatus of the introductorily stated type which, according to the invention, is characterised in that it comprises
an essentially tubular casing arranged to constitute a section of the pipeline proper, a spin element for rotation of the fluid flow being located at the upstream end of the casing, and the outlet means for the central zone comprising a discharge element arranged downstream of the spin element and having entry openings for discharge of the light fraction and possibly entrained heavier fraction from the central zone,
a control separator connected to the discharge element and arranged to separate entrained heavier fraction from the light fraction, the separator being provided with an outlet for separated heavier fraction, and an outlet for the light fraction, and
a control system comprising a level transmitter for indication of the level of separated heavier fraction in the separator, and a level control unit connected to the level transmitter and to a drain valve in the outlet of the separator for the light fraction, and in cooperation with the valve seeing that the separated heavier fraction in the separator being kept at a constant level corresponding to the maximum allowed, entrained quantity of the heavier fraction in the light fraction.
With the present invention there is provided an apparatus which uses in-line technology to separate a fluid flow in a pipeline. The apparatus is based on the cyclone principle and preferably will be provided with axial spin elements simplifying the in-line method in that the supplied fluid flow and at least one of the separated fluid fractions may flow in the pipeline direction axially into and out of the tubular casing. In this manner the casing may be simply installed in a straight pipeline stretch without any substantial modification of the existing pipe geometry.
The advantages of such a construction, as compared to other analogous separation technology, are:
1. A low weight in relation to e.g. conventional separators and gas scrubbers.
2. Low construction costs in relation to e.g. conventional separators and gas scrubbers.
3. The apparatus may be simply installed in pipe paths having very small physical dimensions. The casing proper in most cases will not have a larger diameter than the connection pipes.
4. The apparatus in most cases can be constructed within the pipe specifications in force without having to take into account the stalled pressure tank code, so that pressure tank protecting equipment may be avoided. Existing technology often suffers from the fact that one does not manage to avoid the pressure tank code.
A relevant application of the apparatus according to the invention will be to separate hydrocarbon gas from produced water on an oil production platform, so that a serious two phase flow (slugging) in the produced water-pipeline is eliminated. The apparatus enables the gas to be separated from the water in a simple and compact manner in addition, the separated gas may be recovered instead of being bunt off in the flare of the platform. The result is saved CO2 expenses.
A very essential possible field of use of the apparatus will be to separate a well flow directly in the flow line from the well, either on the platform in question, under water or down in the well. An axial feed flow into the tubular casing of the apparatus reduces the constructional dimensions considerably and enables an installation which does not have a lager diameter than the flow line proper. This type of application will relieve the gas separation process of the platform The well production is often limited because of the gas capacity of conventional separators. By separating the gas from the well flow upstream of the separator train, the total gas quantity into the separator train will decrease, and the bottleneck is abolished. The gas may be conducted outside of the separator tram directly to e.g. reinjection.
The apparatus according to the invention will also be able to be used to separate free water from oil, e.g. as a dewatering cyclone. A typical application will be an installation in the flow line upstream of the choke valve of the well, where possibly present water often will be in a separate free phase. The advantages will be evident, since large quantities of produced water into the separator train occupy separation capacity and easily form bottlenecks. This is often a great problem on older oil fields where the portion of water may amount to more than 90% of the well flow. By bulk separating this water upstream of the separator train, the bottlenecks will be able to be abolished. The dewatering cyclone will have a great potential installed downhole in an oil well having a high water content A high water content reduces the lifting capacity of the well, often down to a profitless level whereafter the well is shut off. By separating the water from oil downhole, the lifting capacity will be maintained, and a continued production is made possible. By using this technology, the utilisation factor proper of an oil field will be able to be raised considerably.
Seabed installations in flow lines and transport pipes will also be a possibility for this technology.